The development of photochromic or phototropic glass began with U.S. Pat. No. 3,208,860 which teaches the utility of silver halide crystallites in glass bodies to impart photochromic characteristics thereto. The base glass compositions stated to be preferred are encompassed within the R.sub.2 O--Al.sub.2 O.sub.3 --B.sub.2 O.sub.3 --SiO.sub.2 system consisting essentially, in weight percent on the oxide basis, of about 4-26% Al.sub.2 O.sub.3, 4-26% B.sub.2 O.sub.3, and 40-76% SiO.sub.2, wherein R.sub.2 O was selected from the group of 2-8% Li.sub.2 O, 4-15% Na.sub.2 O, 6-20% K.sub.2 O, 8-25% Rb.sub.2 O, and 10-30% Cs.sub.2 O, the sum of those ingredients constituting at least 85% of the total composition. To obtain the desired photochromic behavior, at least one halide is added in a minimum effective amount of 0.2% Cl, 0.1% Br, and 0.08% I and silver is added in at least the minimum indicated proportion of 0.2% where chloride is the effective halide, 0.05% where Br is the effective halide, and 0.03% where iodide is the effective halide. The patent also notes the utility of incorporating small amounts of low temperature reducing agents, such as SrO, FeO, CuO, As.sub.2 O.sub.3, and Sb.sub.2 O.sub.3, to enhance the photochromic properties of the glass.
The greatest commercial application for photochromic glass up to the present time has been the fabrication of spectacle lenses, both as prescription lenses and as non-prescription sunglasses. Prescription lenses, marketed under the trademark PHOTOGRAY by Corning Glass Works, Corning, N.Y., have constituted the largest segment of commercial sales. That glass has the approximate composition tabulated below in weight percent:
SiO.sub.2 --55.6 PA1 B.sub.2 O.sub.3 --16.4 PA1 Al.sub.2 O.sub.3 --8.9 PA1 Li.sub.2 O--2.65 PA1 Na.sub.2 O--1.85 PA1 K.sub.2 O--0.01 PA1 BaO--6.7 PA1 CaO--0.2 PA1 PbO--5.0 PA1 ZrO.sub.2 --2.2 PA1 Ag--0.16 PA1 CuO--0.035 PA1 Cl--0.24 PA1 Br--0.145 PA1 F--0.19 PA1 SiO.sub.2 --55.8 PA1 Al.sub.2 O.sub.3 --6.48 PA1 B.sub.2 O.sub.3 --18.0 PA1 Li.sub.2 O--1.88 PA1 Na.sub.2 O--4.04 PA1 K.sub.2 O--5.76 PA1 ZrO.sub.2 --4.89 PA1 TiO.sub.2 --2.17 PA1 CuO--0.011 PA1 Ag--0.24 PA1 Cl--0.20 PA1 Br--0.13 PA1 SiO.sub.2 --54-66 PA1 Al.sub.2 O.sub.3 --7-16 PA1 B.sub.2 O.sub.3 --10-30 PA1 Na.sub.2 O--3-15 PA1 Li.sub.2 O--0-4 PA1 K.sub.2 O--0-10 PA1 PbO--0.4-1.5 PA1 Br--0.2-0.5 PA1 Cl--0.5-1.2 PA1 F--0.2-0.5 PA1 CuO--0.008-0.03 PA1 Ag--&gt;0.03-1 PA1 SiO.sub.2 --55-60 PA1 Al.sub.2 O.sub.3 --9-10 PA1 B.sub.2 O.sub.3 --19-20.5 PA1 Li.sub.2 O--2-2.5 PA1 Na.sub.2 O--2-3 PA1 K.sub.2 O--6-7 PA1 PbO--0.1-0.25 PA1 Ag--0.1-0.15 PA1 Cl--0.3-0.5 PA1 Br--0.05-0.15 PA1 CuO--0.0065-0.01 PA1 (a) a clear luminous transmittance in excess of 65%; PA1 (b) a darkened luminous transmittance at 20.degree. C. of below 25% when the glass is free from tint and below 20% when the glass is tinted to a level of at least 75%, and a fading rate such that the glass demonstrates a luminous transmittance of at least twice that of the darkened transmittance after a five-minute fading interval from the darkened state; and PA1 (c) a darkened luminous transmittance at 40.degree. C. of below 45% when the glass is free from tint and below 40% when the glass is tinted to a level of at least 75%, and a fading rate such that the glass demonstrates a luminous transmittance of at least 1.75 times that of the darkened transmittance after a five-minute fading interval from the darkened state. PA1 (1) a glass-forming batch of the proper composition is melted; PA1 (2) the temperature of at least one region of the glass melt is adjusted to provide a viscosity therein of about 10.sup.4 -10.sup.6 poises; PA1 (3) the glass melt at a viscosity of about 10.sup.4 -10.sup.6 poises is drawn past forming means to produce potentially photochromic drawn glass sheet of optical quality; and then PA1 (4) the glass sheet or an article cut therefrom is exposed to a temperature between about 650.degree.-675.degree. C. for a sufficient length of time to develop photochromic behavior therein. PA1 (a) the glass sheet will be cooled below the softening point of the glass and articles of desired geometries cut therefrom; PA1 (b) the articles will be edge supported on alveolated molds; and then PA1 (c) the articles will be subjected to a temperature between about 650.degree.-675.degree. C. for a sufficient length of time to simultaneously sag the glass to a desired curvature into the concave portions of the alveolated molds, but not into contact with the inner surface thereof, and develop photochromic behavior in the glass.
Inasmuch as the composition of PHOTOGRAY brand lenses comprised compromises struck between photochromic behavior, ophthalmic requirements, glass melting and forming capabilities, and the like, research was continuously undertaken to develop a glass manifesting improved photochromic characteristics, while retaining the other chemical and physical properties required in the manufacture of ophthalmic products.
Recently, prescription lenses have been marketed by Corning Glass Works under the trademark PHOTOGRAY EXTRA which darken to a greater degree and fade more rapidly than the PHOTOGRAY brand lenses. The glass for those lenses is encompassed within U.S. Pat. No. 4,190,451 and consists essentially, expressed in terms of weight percent on the oxide basis, of about:
Ophthalmic lenses prepared from the above two compositions have been formed in a manner similar to that employed in the production of conventional, i.e., non-photochromic, ophthalmic lenses. Thus, a glass blank is pressed and the blank then ground and polished to the desired prescription. To develop photochromic properties, the glass blank is subjected to a predetermined heat treatment.
U.S. Pat. Nos. 4,018,965 and 4,130,437 observed that the production of non-prescription photochromic glass lenses, for example, sunglass lenses, in large quantities by a process requiring the pressing of blanks followed by grinding and polishing necessarily rendered the cost of such lenses quite high. Both of those patents disclose glass compositions which are potentially photochromic and which can be drawn into glass sheet. Potentially photochromic glass sheet signifies glass sheet which, as drawn, is essentially non-photochromic but which, after exposure to an appropriate heat treatment, will display photochromic behavior. The glass compositions of each patent possess a viscosity at the liquidus of at least 10.sup.4 poises, generally about 10.sup.4 -10.sup.6 poises, and evidence long term stability against devitrification when in contact with platinum metal at temperatures corresponding to glass viscosities in the range of 10.sup.4 -10.sup.6 poises. As defined therein, long term stability against devitrification contemplates the growth of a crystalline layer of not more than 10 microns in thickness at the glass-platinum metal interface after a contact of 30 days. This resistance to crystallization is necessitated by the fact that the preferred sheet forming means comprises the use of a downdraw fusion pipe such as is described in U.S. Pat. Nos. 3,338,696 and 3,682,609 which may be formed from platinum metal or a platinum-clad refractory material. Thus, that method, as well as other commercially-utilized sheet drawing methods, does not provide the very rapid quenching action of conventional pressing processes for forming glass articles. This factor presents an uncertainty regarding the feasibility of producing haze-free, high-darkening, potentially photochromic glass sheet.
The glasses of each disclosure have the capability of being chemically strengthened, when present as sheet in thicknesses between about 1.3-1.7 mm, so as to comply with the Federal Food and Drug Administration (FDA) standards for eyeglass lens safety.
Finally, the preferred, potentially photochromic glass compositions of U.S. Pat. No. 4,130,437 can be simultaneously heated treated to induce photochromic behavior therein and sagged into molds to produce eyeglass lens blanks of the proper curvature, such as is described in U.S. Pat. No. 4,088,470. This latter capability, coupled with the faculty of being amenable to sheet drawing, has rendered the fabrication of sunglass lenses much more rapid and economical.
The operable ranges of glass composition disclosed in U.S. Pat. No. 4,018,965 and the preferred ranges of glass composition recorded in U.S. Pat. No. 4,130,437 are tabulated below in weight percent:
______________________________________ U.S. Pat. No. 4,018,965 U.S. Pat. No. 4,130,437 ______________________________________ SiO.sub.2 54-66 57.1-65.3 Al.sub.2 O.sub.3 7-15 9.6-13.9 B.sub.2 O.sub.3 10-25 12-22 Li.sub.2 O 0.5-4 1-3.5 Na.sub.2 O 3.5-15 3.7-12 K.sub.2 O 0-10 0-5.8 Li.sub.2 O + Na.sub.2 O + K.sub.2 O 6-16 6-15 PbO 0-3 0-1.25 Ag 0.1-1 0.12-0.24 Cl 0.1-1 0.2-1 Br 0-3 0.06-0.25 CuO 0.008-0.16 0.002-0.02 F 0-2.5 0-2.5 ______________________________________
U.S. Pat. No. 4,168,339 discloses the production of photochromic glass microsheet, i.e., sheet glass having thicknesses of about 0.25-0.5 mm. The patent utilizes the sheet drawing proces delineated in U.S. Pat. Nos. 3,338,696 and 3,682,609, supra, and comprehends the ranges of glass compositions set out below, expressed in weight percent on the oxide basis:
With respect to photochromic behavior, the glasses of U.S. Pat. No. 4,018,965 are stated to exhibit, at ambient temperatures, i.e., 20.degree.-25.degree. C., a clear luminous transmittance of at least 60%, a darkened luminous transmittance not exceeding 25%, and a fading rate such that the glass manifests a faded luminous transmittance after a 5-minute fading interval from the darkened state of at least 1.5 times that of the darkened transmittance. The glasses of U.S. Pat. No. 4,130,437 are reported to demonstrate at temperatures of 20.degree.-25.degree. C. a clear luminous transmittance of at least 60%, a darkened luminous transmittance below 30%, and a fading rate such that the glass evidences a faded luminous transmittance after a 5-minute fading interval from the darkened state of at least 1.75 times that of the darkened transmittance. Furthermore, after a one-hour fading period, the glass exhibits a luminous transmittance in excess of 80% of its clear luminous transmittance. The glasses of U.S. Pat. No. 4,168,339 are asserted to evidence at room temperature a darkened luminous transmittance below 50% and a fading rate such that, after a 5-minute fading interval, the transmittance will have increased by at least 20 percentage units of transmission and, after a one-hour fading period, the glass will manifest a luminous transmittance in excess of 80%. It will be appreciated that the initial clear luminous transmittance values of the glasses are in the vicinity of 90%. Tinting agents, e.g., CoO and NiO, are added to the glass composition to reduce the initial clear luminous transmittance values to levels approaching 60%. So little tinting agent is required that the photochromic properties of the glass are not substantially affected.
Whereas the photochromic behavior of those disclosed glasses is quite desirable, research has continued to seek out glasses manifesting the melting and forming characteristics of those patented compositions but which would display still further improved photochromic properties, i.e., the glasses will exhibit a deeper darkening transmittance and a more rapid fade rate. Hence, the glasses sought will demonstrate the faculty of being formed into sheet, will show long term stability against devitrification at temperatures corresponding to glass viscosities in the range of 10.sup.4 -10.sup.6 poises, can be simultaneously heat treated to develop photochromic properties therein and sagged into lenses of the proper curvature, will be chemically strengthenable to meet the FDA requirements for eyeglass lens safety, and will manifest the following photochromic properties: (a) a clear luminous transmittance in excess of 65%; (b) a darkened luminous transmittance at 20.degree. C. below 25%; (c) a fading rate at 20.degree. C. such that the glass demonstrates a faded luminous transmittance of at least twice that of the darkened transmittance after a five-minute fading interval from the darkened state; (d) a darkened transmittance at 40.degree. C. below 45%; and (e) a fading rate at 40.degree. C. such that the glass exhibits a faded luminous transmittance of at least 1.75 times that of the darkened transmittance after a five-minute fading interval from the darkened state. (Again, the clear luminous transmittance of the glasses will be in the vicinity of 90% without conventional tinting agents.)